Library docs: Document the order for sort and sortOn.
Library docs: Document the order for sort and sortOn.

This closes GH-22.

shlevy/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,ezyang/ghc